Semiconductor module and method of manufacturing semiconductor module

ABSTRACT

A semiconductor module includes: a semiconductor device; a bonding layer that is arranged on the semiconductor device, and contains nickel or copper, an entire back surface of the bonding layer being electrically connected to and in direct contact with an electrode in the semiconductor device; an anti-oxidation layer disposed on the bonding layer; and a protective layer disposed directly on a top surface of a peripheral portion of the bonding layer on which the anti-oxidation layer is absent, covering an outer peripheral edge of the bonding layer, wherein the protective layer is made of an electrically insulating resin.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a semiconductor module and a method of manufacturing the semiconductor module.

Background Art

Conventionally, semiconductor modules that include semiconductor devices are used. Such semiconductor modules include a semiconductor device and an insulating substrate which supports the semiconductor device, and wires which connect the semiconductor device to an external unit are solder-bonded to electrodes of the semiconductor device.

The semiconductor module includes a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated-gate bipolar transistor (IGBT), a freewheeling diode (FWD), or the like as the semiconductor device, for example.

Accordingly, semiconductor modules are subjected to thermal cycling tests that simulate the usage environment in order to evaluate reliability.

In a semiconductor module, one of the locations upon which stress acts as a result of thermal cycling is a protective layer that protects the electrodes to which wires that connect the semiconductor device to an external unit are solder-bonded.

The electrodes of the semiconductor device and the wires that are connected to those electrodes are connected using solder. Moreover, the area surrounding the electrodes is covered by the protective layer. The protective layer is made of a resin, while the electrodes to which the protective layer adheres are made of metal, and therefore the degree of thermal expansion of both materials is different.

The electrodes of the semiconductor device include a bonding layer which is connected to the circuits of the semiconductor device and an anti-oxidation layer which is arranged on the bonding layer and prevents the bonding layer from oxidizing.

For example, the bonding layer is made of nickel, the anti-oxidation layer is made of gold, and the protective layer is made of a polyimide. In the electrodes to which wires are solder-bonded, the anti-oxidation layer portions are covered by the protective layer.

The polyimide that is used for the protective layer exhibits low adhesive strength with the gold forming the anti-oxidation layer, and therefore subjecting the semiconductor device to thermal cycling tests creates stress on the plane of adhesion between the anti-oxidation layer and the protective layer, which can cause the protective layer to peel from the anti-oxidation layer and result in reduced insulation.

Therefore, Patent Document 1, for example, proposes interposing an amorphous silicon film which exhibits high adhesive strength with polyimides between a polyimide forming a protective layer and an aluminum film forming an electrode in order to improve the adhesive strength between the polyimide and the aluminum film.

Moreover, Patent Document 2 proposes interposing a hexamethyldisilane film which exhibits high adhesive strength with polyimides between a polyimide forming a protective layer and an aluminum film forming an electrode in order to improve the adhesive strength between the polyimide and the aluminum film.

RELATED ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Application Laid-Open Publication     No. 2000-277512 -   Patent Document 2: Japanese Patent Application Laid-Open Publication     No. 2001-189309

SUMMARY OF THE INVENTION

In the technologies proposed in Patent Documents 1 and 2 as described above, when the material to which the polyimide adheres in the electrodes of the semiconductor device is a material other than aluminum, adhesion of the polyimide can potentially suffer.

The present specification aims to provide a semiconductor module including a protective layer which exhibits high bonding strength with an electrode.

Additional or separate features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect, the present disclosure provides a semiconductor module, comprising: a semiconductor device; a bonding layer that is arranged on the semiconductor device, contains nickel or copper, and is electrically connected to the semiconductor device; a solder portion containing gold, disposed on the bonding layer; and a protective layer disposed directly on the bonding layer, covering an outer peripheral edge of the bonding layer.

In this semiconductor module, an outer peripheral edge of the solder portion may coincide with an inner peripheral edge of the protective layer.

Moreover, in this semiconductor module, an outer peripheral edge of the solder portion may be positioned further inwards than an inner peripheral edge of the protective layer.

In particular, in this semiconductor module, between the outer peripheral edge of the solder portion and the inner peripheral edge of the protective layer, the bonding layer may be exposed.

Furthermore, in this semiconductor module, an arithmetic average roughness of a surface of a portion of the bonding layer on which the protective layer may be arranged be greater than or equal to 1 μm and less than or equal to 6 μm.

In addition, in this semiconductor module, the protective layer may contain polyimide or polyamide.

In another aspect, the present disclosure provides a method of manufacturing a semiconductor module, comprising: forming, on a semiconductor device, a bonding layer containing nickel or copper so as to be electrically connected to the semiconductor device; forming an anti-oxidation layer that contains gold on the bonding layer except for on an outer peripheral portion of the bonding layer including an outer peripheral edge of the bonding layer; forming a protective layer directly on the bonding layer so as to cover the outer peripheral edge of the bonding layer; and bonding a region of the bonding layer on which the anti-oxidation layer is formed to a wire using solder.

In this method of manufacturing a semiconductor module, the forming of the anti-oxidation layer may include: forming a layer that contains gold over an entire surface of the bonding layer; and removing a portion of said layer containing gold that is on the outer peripheral portion of the bonding layer so as to form the anti-oxidation layer. Further, the removing of the portion of said layer containing gold that is on the outer peripheral portion of the bonding layer may be performed so as to cause an exposed surface of the outer peripheral portion of the bonding layer to have an arithmetic average roughness of greater than or equal to 1 μm and less than or equal to 6 μm.

In this method of manufacturing a semiconductor module, the anti-oxidation layer and the protective layer may be formed so that an outer peripheral edge of the anti-oxidation layer coincides with as an inner peripheral edge of the protective layer.

The semiconductor module disclosed in the present specification as described above includes a protective layer which exhibits high bonding strength with an electrode.

Moreover, the method of manufacturing a semiconductor module disclosed in the present specification as described above makes it possible to produce a semiconductor module including a protective layer which exhibits high bonding strength with an electrode. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.

In another aspect, the present disclosure provides a semiconductor module, comprising: a semiconductor device; a bonding layer that is arranged on the semiconductor device, and contains nickel or copper, an entire back surface of the bonding layer being electrically connected to and in direct contact with an electrode in the semiconductor device; an anti-oxidation layer disposed on the bonding layer; and a protective layer disposed directly on a top surface of a peripheral portion of the bonding layer on which the anti-oxidation layer is absent, covering an outer peripheral edge of the bonding layer, wherein the protective layer is made of an electrically insulating resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of an embodiment of a semiconductor module disclosed in the present specification, and FIG. 1B is a cross-sectional view taken along line X-X′.

FIG. 2 is a drawing for explaining bonding layers.

FIGS. 3A and 3B illustrate a modification example of the semiconductor module disclosed in the present specification.

FIG. 4 illustrates a (first) step in an embodiment of a method of manufacturing the semiconductor module disclosed in the present specification.

FIG. 5 illustrates a (second) step in the embodiment of the method of manufacturing the semiconductor module disclosed in the present specification.

FIG. 6 illustrates a (third) step in the embodiment of the method of manufacturing the semiconductor module disclosed in the present specification.

FIG. 7 illustrates a (fourth) step in the embodiment of the method of manufacturing the semiconductor module disclosed in the present specification.

FIG. 8 illustrates a (fifth) step in the embodiment of the method of manufacturing the semiconductor module disclosed in the present specification.

FIG. 9 illustrates a (sixth) step in the embodiment of the method of manufacturing the semiconductor module disclosed in the present specification.

FIG. 10 illustrates a (seventh) step in the embodiment of the method of manufacturing the semiconductor module disclosed in the present specification.

DETAILED DESCRIPTION OF EMBODIMENTS

Next, preferred embodiments of a semiconductor module to be disclosed in the present specification will be described with reference to figures. Note, however, that the technical scope of the present invention is not limited to these embodiments and includes both the invention as set forth in the claims as well as equivalents thereof.

FIG. 1A is a plan view of an embodiment of a semiconductor module disclosed in the present specification, and FIG. 1B is a cross-sectional view taken along line X-X′.

A semiconductor module 10 includes a semiconductor device 11 and an insulating substrate 12 which supports the semiconductor device 11. The semiconductor module 10 includes a MOSFET as the semiconductor device 11. Wires 20 a and 20 b which electrically connect the semiconductor device 11 to an external unit are solder-bonded to the semiconductor device 11. Moreover, the semiconductor module 10 may be housed within a casing (not illustrated in the figures). Note that in FIG. 1A, the wires are not explicitly depicted.

The semiconductor device 11 includes a main device body 11 a, a source electrode 11 b, a gate electrode 11 c, and a drain electrode 11 d. The source electrode 11 b and the gate electrode 11 c are arranged on a first surface s1 of the main device body 11 a, and the drain electrode 11 d is arranged on a second surface s2 of the main device body 11 a.

Moreover, on the first surface s1, the semiconductor module 10 includes a first protective layer 15 which covers portions other than where the source electrode 11 b and the gate electrode 11 c are arranged. The first protective layer 15 is electrically insulating and is made of a polyimide or a polyamide, for example.

The insulating substrate 12 supports the semiconductor device 11 and dissipates heat generated by the semiconductor device 11. The insulating substrate 12 includes a main substrate 12 a, a circuit substrate 12 b, and a metal plate 12 c.

The main substrate 12 a is electrically insulating and thermally conductive and provides mechanical strength for supporting the semiconductor device 11. The main substrate 12 a is made of a ceramic such as aluminum nitride, for example.

Via a solder layer 12 d, the circuit substrate 12 b is electrically connected to the drain electrode 11 d and bonds the semiconductor device 11 to the insulating substrate 12. Here, the circuit substrate 12 b may be a copper plate which electrically connects wiring (not illustrated in the figure) to the drain electrode 11 d. The circuit substrate 12 b is electrically connected to a unit external to the semiconductor device 11 via wiring (not illustrated in the figure).

The metal plate 12 c is bonded to a chassis or the like (not illustrated in the figure) to fix the semiconductor module 10 to the chassis and also conducts heat generated by the semiconductor module 10 to the chassis. The metal plate 12 c is made of a metal with high thermal conductivity such as copper, for example.

The dimensions of the main substrate 12 a can be determined as appropriate based on the dimensions of the semiconductor device 11. For example, when the dimensions of the semiconductor device 11 are 10 mm in length by 10 mm in width by 1 mm in thickness, the dimensions of the main substrate 12 a may be set to 30 mm in length by 30 mm in width by 0.5 mm in thickness, and the dimensions of the metal plate 12 c may be set to 25 mm in length by 25 mm in width by 1 mm in thickness.

The semiconductor module 10 includes a first electrode structure 10 a which electrically connects the source electrode 11 b of the semiconductor device 11 and the wire 20 a as well as a second electrode structure 10 b which electrically connects the gate electrode 11 c of the semiconductor device 11 and the wire 20 b. The outer periphery of the source electrode 11 b is covered by the protective layer 15, and an interior portion which is not covered by the protective layer 15 forms a recess. Similarly, the gate electrode 11 c is arranged within a recess surrounded by the protective layer 15. The outer peripheries of the source electrode 11 b and the gate electrode 11 c may be covered by the protective layer 15 but do not necessarily need to be covered. The outlines of the recesses may take any shape, including circular shapes, but from a practical perspective it is preferable that these shapes be rectangular when viewed in a plan view. Here, the outlines of the recesses will be described as being rectangular in shape. Similarly, a bonding layer 14 a and a bonding layer 14 b (described later) may also take any shape, including circular shapes, but from a practical perspective it is preferable that these shapes be rectangular when viewed in a plan view, and therefore here these shapes will be described as being rectangular.

Next, the first electrode structure 10 a will be described below. The first electrode structure 10 a includes the bonding layer 14 a and a solder portion 17 a. The bonding layer 14 a is electrically conductive and is electrically connected to inside of the semiconductor device 11 via the source electrode 11 b. The bonding layer 14 a has a rectangular shape when viewed in a plan view and is arranged on top of and near the center of the source electrode 11 b. The source electrode 11 b is made of aluminum or an aluminum alloy, for example.

It is preferable that the thickness of the source electrode 11 b be in the range of 3 μm to 6 μm. Setting the thickness of the source electrode 11 b to at least 3 m makes it possible to achieve sufficient electrical properties and mechanical strength as a conductor layer. Moreover, if the thickness of the source electrode 11 b is greater than 6 μm, warping of the source electrode 11 b can result in stress on the main device body 11 a.

The bonding layer 14 a of the first electrode structure 10 a is arranged on the source electrode 11 b and is electrically connected to the source electrode 11 b. The bonding layer 14 a is interposed in the solder bond between the source electrode 11 b and the solder portion 17 a. The bonding layer 14 a is rectangular when viewed in a plan view, and the periphery thereof is covered by the protective layer 15. The bonding layer 14 a is made of nickel, a nickel alloy, copper, or a copper alloy.

As illustrated in FIG. 2 , the bonding layer 14 a includes a first region 14 a 1 which is arranged along the outer peripheral edge and a second region 14 a 2 which is arranged on the inner side of the first region 14 a 1. In the semiconductor module 10, the first region 14 a 1 and the second region 14 a 2 contact one another. In other words, the first region 14 a 1 is a region arranged along the outer peripheral edge of the bonding layer 14 a, and the second region 14 a 2 is the region on the inner side of the first region 14 al.

The first region 14 a 1 is a region covered by a protective layer 16 (described later). The second region 14 a 2 is the region in which the solder portion 17 a is arranged.

The first region 14 a 1 has a prescribed width and is arranged along the outer peripheral edge of the bonding layer 14 a. From the perspective of bonding the second protective layer 16 to the bonding layer 14 a with sufficient bonding strength, it is preferable that the width of the first region 14 a 1 be at least 30 μm. The upper limit for the width of the first region 14 a 1 can be determined as appropriate based on the dimensions of the bonding layer 14 a or the solder portion 17 a.

Moreover, from the perspective of bonding the second protective layer 16 (described later) with sufficient strength to the bonding layer 14 a by virtue of mating with irregularities in the surface of the first region 14 a 1, it is preferable that the arithmetic average roughness of the surface of the first region 14 a 1 be greater than or equal to 1 μm and less than or equal to 6 μm.

It is preferable that the thickness of the bonding layer 14 a be in the range of 3 μm to 6 μm. Setting the thickness of the bonding layer 14 a to at least 3 μm makes it possible to achieve sufficient electrical properties and mechanical strength as a conductor layer. Moreover, if the thickness of the bonding layer 14 a is greater than 6 μm, warping of the bonding layer 14 a can result in stress on the semiconductor device 11.

The solder portion 17 a of the first electrode structure 10 a is arranged on the second region 14 a 2 of the bonding layer 14 a and is electrically connected to the bonding layer 14 a. Moreover, the solder portion 17 a electrically connects the bonding layer 14 a and the wire 20 a. The solder portion 17 a is made of a solder containing gold. Furthermore, the solder portion 17 a may be an alloy into which the material forming the bonding layer 14 a mixes when soldered to the bonding layer 14 a.

Next, the second electrode structure 10 b will be described below. The second electrode structure 10 b includes the bonding layer 14 b and a solder portion 17 b. The bonding layer 14 b is electrically conductive and is electrically connected to the semiconductor device 11 via the gate electrode 11 c. The bonding layer 14 b has a rectangular shape when viewed in a plan view and has the same outline as the gate electrode 11 c. The gate electrode 11 c is made of aluminum or an aluminum alloy, for example. The above description of the thickness of the source electrode 11 b of the first electrode structure 10 a can also be applied to the gate electrode 11 c as appropriate.

The bonding layer 14 b of the second electrode structure 10 b is arranged on the gate electrode 11 c and is electrically connected to the gate electrode 11 c. The bonding layer 14 b is interposed in the solder bond between the gate electrode 11 c and the solder portion 17 b. The bonding layer 14 b has a rectangular shape when viewed in a plan view and has the same outline as the gate electrode 11 c. The bonding layer 14 b is made of nickel, a nickel alloy, copper, or a copper alloy.

As illustrated in FIG. 2 , the bonding layer 14 b includes a first region 14 b 1 which is arranged along the outer peripheral edge and a second region 14 b 2 which is arranged on the inner side of the first region 14 b 1.

The first region 14 b 1 is a region covered by the protective layer 16. The second region 14 b 2 is the region in which the solder portion 17 b is arranged.

The above description of the first region 14 a 1 and the second region 14 a 2 of the bonding layer 14 a of the first electrode structure 10 a can also be applied to the first region 14 b 1 and the second region 14 b 2 of the second electrode structure 10 b as appropriate.

Moreover, the above description of the thickness of the bonding layer 14 a of the first electrode structure 10 a can also be applied to the bonding layer 14 b of the second electrode structure 10 b as appropriate.

The solder portion 17 b of the second electrode structure 10 b is arranged on the second region 14 b 2 of the bonding layer 14 b and is electrically connected to the bonding layer 14 b. Moreover, the solder portion 17 b electrically connects the bonding layer 14 b and the wire 20 b. The solder portion 17 b is made of a solder containing gold. Furthermore, the solder portion 17 b may be an alloy into which the material forming the bonding layer 14 b mixes when soldered to the bonding layer 14 b.

The semiconductor module 10 includes the second protective layer 16, which covers a region excluding the solder portions 17 a and 17 b on the first surface s1 of the semiconductor device 11.

The second protective layer 16 is arranged directly on the first region 14 a 1 of the bonding layer 14 a of the first electrode structure 10 a and covers the outer peripheral edge of the bonding layer 14 a. A second region 14 a 2-side edge 16 a of the second protective layer 16 is arranged on the first region 14 a 1 and contacts an edge 17 a 1 of the solder portion 17 a. In other words, the inner peripheral edge of the second protective layer 16 contacts the outer peripheral edge of the solder portion 17 a. As illustrated by the hatching in FIG. 1A, the second protective layer 16 bonds (adheres) to the surface of the first region 14 a 1 via a bonding surface 16 a 1 between the second protective layer 16 and the first region 14 a 1. The bonding surface 16 a 1 has a ring-shaped shape which surrounds the solder portion 17 a.

Moreover, the second protective layer 16 is arranged directly on the first region 14 b 1 of the bonding layer 14 b of the second electrode structure 10 b and covers the outer peripheral edge of the bonding layer 14 b. A second region 14 b 2-side edge 16 b of the second protective layer 16 is arranged on the first region 14 b 1 and contacts an edge 17 b 1 of the solder portion 17 b. In other words, the inner peripheral edge of the second protective layer 16 contacts the outer peripheral edge of the solder portion 17 b. As illustrated by the hatching in FIG. 1A, the second protective layer 16 bonds (adheres) to the surface of the first region 14 b 1 via a bonding surface 16 b 1 between the second protective layer 16 and the first region 14 b 1. The bonding surface 16 b 1 has a ring-shaped shape which surrounds the solder portion 17 b.

The second protective layer 16 is electrically insulating and is made of a polyimide or a polyamide, for example.

In the semiconductor module according to the present embodiment as described above, the respective bonding layers of the first electrode structure and the second electrode structure exhibit high bonding strength with the second protective layer, thereby preventing the second protective layer from peeling from the bonding layers. Therefore, the first electrode structure and the second electrode structure are protected by the second protective layer, and the electrical insulation properties are also maintained, which improves the reliability of the semiconductor module.

When the semiconductor device undergoes warping as a result of temperature changes or the like in the semiconductor module 10, for example, shear stress can occur between the second protective layer and the surfaces of the first regions of the bonding layers. The second protective layer is bonded (adhered) to the surfaces of the first regions by the bonding surfaces between the second protective layer and the first regions, and therefore the shear stress applied per unit area of the bonding surface between the second protective layer and the first regions of the bonding layers is small, which makes the second protective layer less prone to peeling from the first surfaces of the bonding layers.

Next, a modification example of the semiconductor module according to the present embodiment as described above will be described below with reference to FIGS. 3A and 3B.

FIGS. 3A and 3B illustrates a modification example of the semiconductor module disclosed in the present specification. In a semiconductor module 10 according to the present modification example, in the first electrode structure 10 a the second region 14 a 2-side edge 16 a of the second protective layer 16 and the edge 17 a 1 of the solder portion 17 a are separated from one another, and the bonding layer 14 a is exposed between these edges. In other words, the inner peripheral edge of the second protective layer 16 and the outer peripheral edge of the solder portion 17 a are separated, and the bonding layer 14 a is exposed between these edges. This is to say that the first region 14 a 1 and the second region 14 a 2 are separated from one another.

In this case, in the region in which the bonding layer 14 a is exposed, the bonding layer 14 a is exposed to the atmosphere and undergoes oxidation, which inhibits wetting and spreading of solder.

During manufacture of the semiconductor module 10, arranging the second region 14 a 2-side edge 16 a of the second protective layer 16 and the edge 17 a 1 of the solder portion 17 a to be separated from one another (that is, arranging the inner peripheral edge of the second protective layer 16 and the outer peripheral edge of the solder portion 17 a to be separated from one another) makes it possible to allocate margin in terms of placement accuracy when forming the second protective layer 16 and/or the solder portion 17 a. This makes it possible to reduce the time and cost of the manufacturing process.

Moreover, in the second electrode structure 10 b, the second region 14 b 2-side edge 16 b of the second protective layer 16 and the edge 17 b 1 of the solder portion 17 b are separated from one another, and the bonding layer 14 b is exposed between these edges. In other words, the inner peripheral edge of the second protective layer 16 and the outer peripheral edge of the solder portion 17 b are arranged so as to be separated from one another, which achieves the same advantageous effect as described above. The rest of the configuration of the present modification example is the same as in the embodiment described above.

Next, a preferred embodiment of a method of manufacturing the semiconductor module described above will be described below with reference to FIGS. 4 to 10 .

First, as illustrated in FIG. 4 , a semiconductor device 11 including a source electrode 11 b, a gate electrode 11 c, and a drain electrode 11 d is prepared. Aluminum or an aluminum alloy can be used as the material for the source electrode 11 b and the gate electrode 11 c, for example. The source electrode 11 b and the gate electrode 11 c may be 5 m in thickness, for example. Although only a single semiconductor device 11 is depicted in FIG. 4 , the semiconductor device 11 is prepared in a form in which a plurality of semiconductor devices 11 are arranged on a single wafer. The following steps will be described for a single semiconductor device 11, but the same processes are also applied to the other semiconductor devices.

Next, as illustrated in FIG. 5 , a first protective layer 15 is formed on the semiconductor device 11, thereby yielding a semiconductor structure 30. The first protective layer 15 is formed on a first surface s1 of the semiconductor device 11 so as to leave the source electrode 11 b and the gate electrode 11 c exposed. The first protective layer 15 may cover the outer peripheries of the source electrode 11 b and the gate electrode 11 c. Whether the first protective layer 15 covers or does not cover the outer peripheries of the source electrode 11 b and the gate electrode 11 c may be selected as appropriate on the basis of the sizes of the source electrode 11 b and the gate electrode 11 c. Here, a case in which the outer periphery of the source electrode 11 b is covered by the first protective layer 15 while the outer periphery of the gate electrode is not covered by the first protective layer 15 will be described. The first protective layer 15 is formed, for example, by applying a liquid polyimide or polyamide to the first surface s1 of the semiconductor device 11 so as to leave the source electrode 11 b and the gate electrode 11 c exposed and then applying heat to harden this material.

Next, as illustrated in FIG. 6 , a bonding layer 14 a is formed on the source electrode 11 b, and a bonding layer 14 b is formed on the gate electrode 11 c. The bonding layer 14 a and the bonding layer 14 b are formed using a plating technology such as electroless plating, for example. It is preferable that the bonding layer 14 a and the bonding layer 14 b be made of nickel, a nickel alloy, copper, or a copper alloy. More specifically, the bonding layer 14 a and the bonding layer 14 b may be formed as nickel phosphorus films using an electroless plating technology. The thickness of the bonding layer 14 a and the bonding layer 14 b may be set to 5 μm, for example.

Next, as illustrated in FIG. 7 , an anti-oxidation layer 18 a is formed on the bonding layer 14 a, and an anti-oxidation layer 18 b is formed on the bonding layer 14 b. The anti-oxidation layer 18 a and the anti-oxidation layer 18 b prevent the bonding layer 14 a and the bonding layer 14 b from oxidizing. The anti-oxidation layer 18 a and the anti-oxidation layer 18 b are formed as gold or gold alloy films using a plating technology such as electroless plating, for example. It is preferable that the thickness of the anti-oxidation layer 18 a and the anti-oxidation layer 18 b be set in the range of 0.025 m to 0.1 m. A thickness of 0.025 m ensures a thickness that makes it possible to prevent the bonding layer 14 a and the bonding layer 14 b from oxidizing. Moreover, a thickness of no more than 0.1 μm makes it possible to reduce material costs and maintain a uniform thickness.

Next, as illustrated in FIG. 8 , the anti-oxidation layer 18 a on a first region 14 a 1 of the bonding layer 14 a is removed, and the first region 14 a 1 of the bonding layer 14 a is exposed. Similarly, the anti-oxidation layer 18 b on a first region 14 b 1 of the bonding layer 14 b is removed, and the first region 14 b 1 of the bonding layer 14 b is exposed. More specifically, a first mask (not illustrated in the figure) is formed on the first surface s1 of the semiconductor device 11 so as to leave only the anti-oxidation layer 18 a on the first region 14 a 1 of the bonding layer 14 a and the anti-oxidation layer 18 b on the first region 14 b 1 of the bonding layer 14 b exposed, and the portions of the anti-oxidation layer 18 that are exposed from the first mask are removed by etching. A wet etching process or a dry etching process such as plasma etching can be used for this etching. Moreover, as an alternative to etching, a blasting process may be used to remove the portions of the anti-oxidation layer 18 on the bonding layer 14 a and the bonding layer 14 b that are exposed from the mask. Alternatively, laser irradiation may be used to remove the portion of the anti-oxidation layer 18 a on the first region 14 a 1 of the bonding layer 14 a and the portion of the anti-oxidation layer 18 b on the first region 14 b 1 of the bonding layer 14 b.

It is preferable that the etching conditions, the particle size of particles used in a blasting process, the laser irradiation conditions, or the like described above be adjusted such that the arithmetic average roughness of the surface of the first region 14 a 1 and the first region 14 b 1 is greater than or equal to 1 μm and less than or equal to 6 μm. This allows a second protective layer 16 to mate with irregularities formed in the surfaces of the first region 14 a 1 and the first region 14 b 1, thereby increasing the bonding strength of the second protective layer 16 with these regions.

Next, as illustrated in FIG. 9 , the second protective layer 16 is formed on the first region 14 a 1 of the bonding layer 14 a so as to cover the outer peripheral edge of the bonding layer 14 a, and the second protective layer 16 is also formed on the first region 14 b 1 of the bonding layer 14 b so as to cover the outer peripheral edge of the bonding layer 14 b. The second protective layer 16 is formed covering the first protective layer 15 on the first surface s1 of the semiconductor device 11 so as to leave only the second region 14 a 2 of the bonding layer 14 a and the second region 14 b 2 of the bonding layer 14 b exposed. Here, it is preferable that the second protective layer 16 be formed so as to not cover onto the anti-oxidation layer 18 a and the anti-oxidation layer 18 b. This is because if the second protective layer 16 covers onto the anti-oxidation layer 18 a and the anti-oxidation layer 18 b, solder portions 17 a and 17 b (described later) become more difficult to bond to the second regions 14 a 2 and 14 b 2 of the bonding layers 14 a and 14 b.

The second protective layer 16 can be formed as follows, for example. In a first method, a second mask (not illustrated in the figures) is formed covering the first mask on the anti-oxidation layer 18 a and the anti-oxidation layer 18 b. Here, the second mask is formed so as to cover a portion of the first region 14 a 1 of the bonding layer 14 a on a second region 14 a 2-side as well as a portion of the first region 14 b 1 of the bonding layer 14 b on a second region 14 b 2-side. Then, the first mask that is not covered by the second mask is removed. Next, with the second region 14 a 2 of the anti-oxidation layer 18 a and the second region 14 b 2 of the anti-oxidation layer 18 b covered by the first mask, a liquid polyimide or polyamide may be applied to the first surface s1 of the semiconductor device 11 using a spin coating process, and then heat may be applied to harden this material and form the second protective layer 16. The heating conditions can be set to one hour at 350° C., for example. Then, the first mask is removed from on the anti-oxidation layer 18 a and the anti-oxidation layer 18 b.

In a second method, after removing the first mask used in the step in FIG. 8 , a photosensitive polyimide or polyamide is applied to the first surface s1 of the semiconductor device 11 and then patterned and temporarily cured to form the second protective layer 16 on the first surface s1 of the semiconductor device 11. Then, the temporarily cured second protective layer 16 may be heated and fully cured to form the second protective layer 16.

In a third method, after removing the first mask used in the step in FIG. 8 a liquid polyimide or polyamide may be applied to a prescribed region on the first surface s1 of the semiconductor device 11 using an inkjet process and may then be heated and fully cured to form the second protective layer 16. Then, the plurality of semiconductor structures 30 formed on the single wafer are cut into individual semiconductor structures 30.

Next, as illustrated in FIG. 10 , in the semiconductor structure 30, the drain electrode 11 d of the semiconductor device 11 is bonded onto a circuit substrate 12 b of an insulating substrate 12 via a solder layer 12 d.

Then, as illustrated in FIGS. 1A and 1B, a solder portion 17 a is formed by using solder to electrically bond a wire 20 a and the second region 14 a 2 of the bonding layer 14 a in which the anti-oxidation layer 18 a is arranged. The anti-oxidation layer 18 a melts into the solder and becomes part of the solder portion 17 a. In other words, once this happens the solder portion 17 a contains gold. Similarly, a solder portion 17 b is formed by using solder to electrically bond a wire 20 b and the second region 14 b 2 of the bonding layer 14 b in which the anti-oxidation layer 18 b is arranged. The anti-oxidation layer 18 b melts into the solder and becomes part of the solder portion 17 b. Completing the steps described above yields the semiconductor module 10. Moreover, when using the solder portion 17 a to bond together the bonding layer 14 a and the wire 20 a, if a portion of the bonding layer 14 a is exposed on the surface, the surface of the bonding layer 14 a is exposed to the atmosphere and undergoes oxidation, which inhibits wetting and spreading of the solder. In this case, the positions of the outer peripheral edge of the solder portion 17 a and the outer peripheral edge of the anti-oxidation layer 18 a match one another. Here, the “portion of the bonding layer 14 a that is exposed on the surface” refers to the region sandwiched between the inner peripheral edge of the protective layer 16 and the outer peripheral edge of the solder portion 17 a (or the outer peripheral edge of the anti-oxidation layer 18 a) in the modification example of the semiconductor module according to the embodiment described above.

In the method of manufacturing the semiconductor module according to the present embodiment as described above, the second protective layer 16 is formed directly on top of the first regions 14 a 1 and 14 b 1 of the bonding layers 14 a and 14 b but is not formed on the second regions 14 a 2 and 14 b 2 of the bonding layers 14 a and 14 b, thereby forming a second protective layer 16 that exhibits high bonding strength with the bonding layers 14 a and 14 b.

In the present invention, the semiconductor module and method of manufacturing a semiconductor module according to the embodiments described above can be modified as appropriate without departing from the spirit of the present invention.

For example, although the semiconductor module according to the embodiments described above includes a MOSFET as a semiconductor device, the semiconductor device is not limited to this example. The semiconductor module may include a semiconductor device such as an IGBT or FWD as the semiconductor device, for example.

Moreover, although in the semiconductor module according to the embodiments described above a bonding layer is arranged on the source electrode of the semiconductor device, an electrode layer made of a conductor may be arranged between the source electrode and the bonding layer. The material and thickness of this conductor may be the same as those used for the source electrode. Similarly, an electrode layer made of a conductor may be arranged between the gate electrode of the semiconductor device and the respective bonding layer. The material and thickness of this conductor may be the same as those used for the gate electrode.

Furthermore, although in the semiconductor module according to the embodiments described above the second protective layer that protects the first electrode structure and the second electrode structure is shared therebetween, protective layers that protect the first electrode structure and the second electrode structure may be separate members.

In addition, although in the method of manufacturing the semiconductor module according to the embodiments described above the protective layer is formed on the first regions of the bonding layers so as to cover the outer peripheral edges of the bonding layers and then the second regions of the bonding layers in which the anti-oxidation layers are arranged are electrically bonded to wires using solder, the order of these steps may be reversed.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents. In particular, it is explicitly contemplated that any part or whole of any two or more of the embodiments and their modifications described above can be combined and regarded within the scope of the present invention. 

What is claimed is:
 1. A semiconductor module, comprising: a semiconductor device; a bonding layer that is arranged on the semiconductor device, and contains nickel or copper, an entire back surface of the bonding layer being electrically connected to and in direct contact with an electrode in the semiconductor device; an anti-oxidation layer disposed on the bonding layer; and a protective layer disposed directly on a top surface of a peripheral portion of the bonding layer on which the anti-oxidation layer is absent, covering an outer peripheral edge of the bonding layer, wherein the protective layer is made of an electrically insulating resin.
 2. The semiconductor module according to claim 1, wherein the anti-oxidation layer contains gold.
 3. The semiconductor module according to claim 1, wherein the semiconductor device includes a source electrode and a gate electrode, both disposed on a first surface of a main device body, and a drain electrode disposed on a second surface of the main device body.
 4. The semiconductor module according to claim 3, further comprising another protective layer disposed directly on the first surface of the main device body so as to surround the source electrode and the gate electrode in a plan view.
 5. The semiconductor module according to claim 4, wherein said another protective layer is disposed between the source electrode and the gate electrode.
 6. A semiconductor module, comprising: a semiconductor device having a source electrode and a gate electrode on a first surface of a main device body thereof, and a drain electrode on a second surface of the main device body; a first bonding layer that is arranged on the source electrode, and contains nickel or copper; a second bonding layer that is arranged on the gate electrode, and contains nickel or copper; a protective layer disposed over the first surface of the semiconductor device so as to surround the source electrode and the gate electrode in a plan view, the protective layer being made of an electrically insulating resin; a first anti-oxidation layer disposed on the first bonding layer; and a second anti-oxidation layer disposed on the second bonding layer, wherein the protective layer is disposed directly on a top surface of a peripheral portion of the first bonding layer on which the first anti-oxidation layer is absent and directly on a top surface of a peripheral portion of the second bonding layer on which the second anti-oxidation layer is absent.
 7. The semiconductor module according to claim 6, wherein each of the first and second anti-oxidation layers contains gold.
 8. The semiconductor module according to claim 6, further comprising another protective layer disposed between the source electrode and the gate electrode in a plan view.
 9. The semiconductor module according to claim 8, wherein said another protective layer is disposed directly on a peripheral portion of the source electrode on which the first bonding layer is absent.
 10. The semiconductor module according to claim 8, wherein the second bonding layer is disposed to cover a substantially entire top surface of the gate electrode.
 11. The semiconductor module according to claim 8, wherein the protective layer and said another protective layer are formed of a same material.
 12. The semiconductor module according to claim 6, wherein the protective layer overlaps the top surface of the peripheral portion of the first bonding by at least 30 m.
 13. The semiconductor module according to claim 6, wherein the protective layer overlaps the top surface of the peripheral portion of the second bonding by at least 30 m.
 14. The semiconductor module according to claim 6, wherein an arithmetic average roughness of a surface of a portion of the first bonding layer on which the protective layer is disposed is greater than or equal to 1 μm and less than or equal to 6 μm, and an arithmetic average roughness of a surface of a portion of the second bonding layer on which the protective layer is disposed is greater than or equal to 1 μm and less than or equal to 6 μm.
 15. The semiconductor module according to claim 6, wherein the protective layer contains polyimide or polyamide.
 16. The semiconductor module according to claim 6, further comprising: a circuit substrate electrically connected to the drain electrode of the semiconductor device; and a ceramic substrate mounting the circuit substrate thereon.
 17. The semiconductor module according to claim 6, wherein a thickness of the source electrode is in the range of 3 μm to 6 μm.
 18. The semiconductor module according to claim 6, wherein a thickness of each of the first and second bonding layers is in the range of 3 μm to 6 μm.
 19. A semiconductor module, comprising: a semiconductor device; a bonding layer that is arranged on the semiconductor device, and contains nickel or copper, an entire back surface of the bonding layer being electrically connected to and in direct contact with an electrode in the semiconductor device; a solder portion disposed on the bonding layer; and a protective layer disposed directly on a top surface of a peripheral portion of the bonding layer on which the solder portion is absent, covering an outer peripheral edge of the bonding layer, wherein the protective layer is made of an electrically insulating resin. 